1. Field of the Invention
This invention relates to an improved method of thermally processing semiconductor wafers and an apparatus therefor. The apparatus improves the manner in which temperature is raised and reduced for thermal processes within a reaction tube.
2. Description of the Prior Art
In the manufacturing process of semiconductor integrated circuits, semiconductor wafers have typically been introduced into a thermal processing apparatus having an atmosphere at a high temperature of about 1000.degree. C. In the apparatus, the semiconductor wafers undergo thermal oxidation and other thermal processes. To enhance the uniformity of the temperature distribution within the apparatus, an heat insulating material is provided in the vicinity of a heater. Further, a tube for distributing heat evenly, made of silicon carbide or the like, is provided between the heater and a process tube. As a result, the heat capacity of the thermal processing apparatus becomes inevitably large. Thus, the apparatus consumes considerable time while raising and reducing temperatures before and after the thermal oxidation or other thermal processing at the prescribed temperatures. This reduces the productivity of semiconductor wafer manufacturing process. Further, the depth of an impurity diffusion layer cannot be controlled to remain small as required.
If the semiconductor wafers are rapidly taken out from the apparatus after the predetermined thermal processes, such as oxidation and impurity diffusion, the semiconductor wafers experience a large temperature change. As a result, the wafer surface is deformed due to "slip" or "warp". Such "slip" or "warp" causes defects in the crystal lattice.
Therefore, in place of the above-described apparatus, a semiconductor wafer thermal processing apparatus utilizing a lamp heater technique has been developed. In the lamp heater technique, the apparatus causes semiconductor wafers to absorb light radiated from a light source provided in the apparatus. As a result, only the wafer temperature is raised. Thus, the heat capacity of the apparatus is significantly smaller than the above-described heater type apparatus. Therefore, the apparatus has advantages in that a speed of raising and reducing temperature is significantly higher. However, the apparatus also has a disadvantage in that the thermal process is performed while the temperature is unstable, in that when the wafer surfaces reach a predetermined temperature, the heater power source is automatically turned off before the interior of the wafer achieves the same temperature as the surface. Also, a speed of raising the temperature varies with the impurity concentration of the semiconductor wafers due to the manner in which the impurities alter the energy absorption of the wafers.
Moreover, there has been also disclosed a thermal processing apparatus based on a combined technique of the heater type and the lamp heater type. Specifically, in such an apparatus, a heater and a lamp are provided between a reflector and a quartz reaction tube. Therefore, the heat capacity of a heater portion constituted by the heater and the lamp is reduced. Thus, a speed of raising and reducing the temperature in the reaction tube is increased. As a result, this compound-type apparatus has a temperature reducing speed higher than that of the heater type thermal processing apparatus. Further, the compound-type apparatus can achieve a temperature raising speed independent of the impurity concentration of the semiconductor wafers to be processed. Therefore, the compound-type apparatus has become recognized as a satisfactory thermal processing apparatus.
However, this apparatus still has a disadvantage as follows. Specifically, when the heater of the apparatus is energized up to its maximum output so as to achieve a thermal process at a higher temperature, the heat capacity of the apparatus increases. Thus, a temperature reducing speed of the apparatus is inevitably reduced. As a result, there has been a compromising relationship between higher-temperature thermal processing and higher-speed temperature reduction.